Controlling Selectivity in the Chlorine Evolution Reaction over RuO₂‐Based Catalysts

Abstract

In the industrially important Chlor-Alkali process, the chlorine evolution reaction (CER) over a ruthenium dioxide (RuO 2 ) catalyst competes with the oxygen evolution reaction (OER). This selectivity issue is elucidated on the microscopic level with the single-crystalline model electrode RuO 2 (110) by employing density functional theory (DFT) calculations in combination with the concept of volcano plots. We demonstrate that one monolayer of TiO 2 (110) supported on RuO 2 (110) enhances the selectivity towards the CER by several orders of magnitudes, while preserving the high activity for the CER. This win-win situation is attributed to the different slopes of the volcano curves for the CER and OER.An in-depth understanding and control of selectivity are great challenges in heterogeneous catalysis research, since the actual outcome of a branched reaction-selectivity-is governed by small energy differences in the activation energies of the competing reaction routes, [1,2] rather than by the absolute activation energies that determine the activity of a specific reaction. Although ab initio theory has developed into a powerful tool in catalysis research to gain insight into the activity of catalyzed reactions at the molecular level, [3][4][5] the selectivity issue is still in its infancy, particularly in electrocatalysis.A prototypical selectivity problem in electrocatalysis is encountered with the Chlor-Alkali process, where the anodic oxidation of brine over RuO 2 -based electrodes leads to the evolution of both oxygen and chlorine. From a thermodynamic point of view, the oxygen evolution reaction (OER: equilibrium potential U o = 1.23 V) is preferred over the chlorine evolution reaction (CER: equilibrium potential U o = 1.36 V). However, since the CER constitutes a twoelectron process and the OER a four-electron process, the kinetics of the CER are much faster than that of the OER, so that the CER prevails above 1.36 V. A mixture of RuO 2 with SnO 2 or TiO 2 has been shown to improve both the stability of the electrode [6] and the selectivity of the anodic reaction towards the CER, [7] although the underlying chemistry is not well understood on a molecular level.Over the past decade, theory [8][9][10][11][12][13][14] has increased our molecular understanding of electrocatalytic reactions through the use of ab initio thermodynamics. [15] Ab initio Pourbaix diagrams [9] reveal stable surface structures under (suppressed) reaction conditions, and universal scaling relationships [3] have been employed to identify the optimum adsorption properties of the electrode material for the reaction under consideration. [8,9] Here, we report the results from ab initio thermodynamics calculations on the selectivity problem of the CER and OER over RuO 2 (110)-based model anodes and show how we can improve the selectivity by coating RuO 2 with a single TiO 2 layer.A Pourbaix diagram indicates the thermodynamically most stable structure of the catalyst surface for a given electrode potential U and pH value. [16] The Pourbaix diagr...

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